The shape of the lesson
By the end of the lesson, learners can
- From the larynx to the alveoli: Rotate the 3D model and click each part.
- Four features of a gas exchange surface: Oxygen diffuses from the air in an alveolus into the blood in the capillary, and carbon dioxide diffuses the other way.
- Keeping the airways clean: Extended.
- Airway order: larynx, trachea, bronchi, bronchioles, alveoli.
- Gas exchange surface: large surface area, thin surface, good blood supply, good ventilation with air.
- Say why: area = more crossing at once; thin = short distance; blood and ventilation = steep concentration gradients.
- Extended: cartilage holds the trachea open; goblet cells make mucus; mucus traps; cilia sweep.
One picture every idea returns to
Air passes through the larynx, trachea, bronchi and bronchioles to the alveoli, tiny air sacs surrounded by capillaries. Gas exchange surfaces in humans have four features: a large surface area, a thin surface, a good blood supply and good ventilation with air. Together they keep oxygen diffusing into the blood and carbon dioxide diffusing out as fast as possible.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Down the airways to 700 million air sacs.. Every breath travels from the larynx down the trachea, bronchi and bronchioles to the alveoli, where oxygen passes into the blood and carbon dioxide passes out. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: From the larynx to the alveoli | Rotate the 3D model and click each part. Project tab 1 of the lab (The breathing system in 3D) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Four features of a gas exchange surface | Oxygen diffuses from the air in an alveolus into the blood in the capillary, and carbon dioxide diffuses the other way. Project tab 3 of the lab (Gas exchange in the alveoli) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Keeping the airways clean | Extended. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 20 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Keeping the airways clean, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The order matters: trachea, then bronchi, then bronchioles, then alveoli. | A common slip is to put bronchioles before bronchi. The singular of bronchi is bronchus, and of alveoli is alveolus. |
| "Thin" refers to the surface (one cell thick), not to the lungs being thin. | And a good blood supply does not push oxygen in: it carries oxygen away so the concentration gradient stays steep. |
| Cilia do not trap anything: the mucus traps, the cilia move the mucus. | Goblet cells make mucus; they are not ciliated. Keep the three jobs separate. |
The breathing system: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Starting from the larynx, name in order the structures that air passes through to reach the blood in the lungs.
Describe four features of the alveoli that make them an efficient gas exchange surface, and explain how each one helps.
In a model of an alveolus, the rate of oxygen uptake with all four features was 10 units per second. The table shows the rate with one feature removed.
| Feature removed | Rate / units per second |
|---|---|
| large surface area | 2.0 |
| thin surface | 3.0 |
| good blood supply | 3.0 |
| good ventilation | 3.5 |
(a) Calculate the rate without a large surface area as a percentage of the full rate. (b) Explain why removing the blood supply reduces the rate.
The disease emphysema breaks down the walls between alveoli, forming fewer, larger air spaces. Suggest why people with emphysema become breathless.
Extended. State the function of the cartilage in the trachea, and state where the internal and external intercostal muscles are found.
Extended. Explain how goblet cells, mucus and ciliated cells protect the breathing system from pathogens and particles. Suggest why smokers, whose cilia are damaged, cough more and get more chest infections.
Answer key and marking notes
- Trachea.
- Bronchus (bronchi).
- Bronchioles.
- Alveoli, then across the alveolus wall and capillary wall into the blood.
- Large surface area (millions of alveoli), so more oxygen and carbon dioxide can diffuse at the same time.
- Thin surface (walls one cell thick), so the diffusion distance is short.
- Good blood supply (dense capillaries), which carries oxygen away and brings carbon dioxide, keeping the concentration gradients steep.
- Good ventilation with air, which brings fresh air and removes carbon dioxide, keeping the gradients steep.
- (a) 2.0 ÷ 10 × 100 = 20%.
- (b) Without flowing blood, oxygen is not carried away from the capillary, so its concentration in the blood rises.
- The concentration gradient between the air in the alveolus and the blood becomes smaller, so less oxygen diffuses in.
- Fewer, larger air spaces have a much smaller total surface area for gas exchange.
- So less oxygen diffuses into the blood per minute, and the person must breathe faster to get enough oxygen.
- Cartilage (in C-shaped rings) supports the trachea and keeps it open, so it does not collapse when the pressure inside falls.
- The external intercostal muscles lie between the ribs on the outer side.
- The internal intercostal muscles lie between the ribs on the inner side.
- Goblet cells secrete mucus onto the lining of the airways.
- The sticky mucus traps dust, particles and pathogens such as bacteria.
- Cilia on ciliated cells beat to sweep the mucus up to the throat, where it is swallowed, so pathogens do not reach the alveoli.
- With damaged cilia the mucus collects in the airways; coughing is the only way to move it, and trapped pathogens stay in the lungs and can cause infections.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
In which part of the breathing system does oxygen pass from the air into the blood?
- Athe alveoli
- Bthe trachea
- Cthe bronchi
- Dthe larynx
The diagram shows part of the breathing system. Which letter labels a bronchus?
- AB
- BA
- CC
- DD
Why is a dense network of capillaries around the alveoli important for gas exchange?
- AFlowing blood keeps the concentration gradients steep
- BCapillaries pump oxygen into the alveoli by force
- CCapillaries make the alveolus walls much thicker
- DBlood in capillaries produces oxygen for the alveoli
The total surface area of the alveoli in an adult is about 70 m2. The surface area of the skin is about 1.75 m2. How many times larger is the alveolar surface?
- A40
- B4
- C68
- D0.025
In pneumonia, the alveoli fill with fluid. Why does this reduce the amount of oxygen entering the blood?
- AThe diffusion distance from air to blood increases
- BThe concentration of oxygen in the blood increases
- CThe capillaries around the alveoli disappear
- DOxygen can no longer move by diffusion at all
Extended: what is the role of goblet cells in the lining of the trachea?
- AThey secrete mucus that traps particles and pathogens
- BThey beat to move mucus up towards the throat
- CThey hold the trachea open during breathing in
- DThey absorb oxygen from the air in the trachea
Extended: which muscles lie between the ribs on the outer side of the ribcage?
- Athe external intercostal muscles
- Bthe internal intercostal muscles
- Cthe diaphragm muscles
- Dthe abdominal muscles
Extended: chemicals in tobacco smoke stop cilia from beating. Which is the most likely result?
- AMucus and trapped pathogens build up in the airways
- BGoblet cells stop producing any mucus at all
- CThe trachea collapses during each breath in
- DMore oxygen diffuses into the blood in the alveoli
Quiz key
Gas exchange happens in the alveoli, whose thin walls are wrapped in capillaries. The trachea and bronchi are airways that carry air to and from the alveoli but have walls far too thick for gas exchange, and the larynx is the voice box at the top of the trachea.
B points to one of the two large branches of the trachea, a bronchus. A is the trachea itself, before it divides. C is a bronchiole, one of the smaller tubes branching inside the lung. D is the diaphragm, the sheet of muscle below the lungs.
Blood flowing through the capillaries carries oxygen away and brings carbon dioxide, so the oxygen concentration in the blood stays lower than in the air, and the gradient stays steep. Capillaries do not pump gases (diffusion is passive), they do not thicken the surface (that would slow diffusion), and blood does not make oxygen.
70 ÷ 1.75 = 40 times larger. 4 slips a decimal place, 68 is the difference (70 − 1.75 rounded) rather than the ratio, and 0.025 is the ratio the wrong way round (1.75 ÷ 70).
A layer of fluid makes the barrier between the air and the blood much thicker, so the diffusion distance increases and oxygen crosses more slowly. It does not raise oxygen in the blood (that would need more oxygen to arrive), the capillaries are still there, and diffusion still happens, only more slowly.
Goblet cells make and release mucus, which traps dust and pathogens. Beating to move mucus is the job of the cilia on ciliated cells, holding the trachea open is the job of the cartilage, and no gas exchange happens in the trachea.
The external intercostal muscles lie on the outer side of the ribs; the internal intercostal muscles lie on the inner side. The diaphragm is a sheet of muscle below the lungs, and the abdominal muscles are in the wall of the abdomen, not between the ribs.
With cilia not beating, mucus is no longer swept up to the throat, so it collects in the airways with its trapped pathogens, causing coughing and chest infections. Smoke actually increases mucus production, the cartilage still holds the trachea open, and gas exchange does not improve.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.